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Side stiches are common. In one study of 965 athletes, side stitches affected 75 percent of swimmers, 69 percent of runners, 62 percent of horse riders, 52 percent of aerobics participants, 47 percent of basketball players and 32 percent of cyclists.

There is no single reason why side stitches occur. The leading theory suggests increased blood flow to the liver and spleen. Another theory is that pain is caused by internal organs pulling down on the diaphragm. (This doesn’t explain why side stitches frequently occur in swimming, though.)

There is also the chance that an imbalance of electrolytes in the blood, such as calcium, potassium and sodium, contributes to side stitches.